1. Field of the Invention
The present invention relates to logic level propagation delay systems, and more particularly, delay systems which avoid spurious signals at an output thereof prior to the full propagation of a logic level to the output.
2. Description of the Prior Art
Digital systems of many kinds are in increasing use. This situation is occurring because of the decrease in cost and because of the increase in performance due to improvements in monolithic integrated circuit technology. Further increases in the use of such digital systems is therefore assured even though many uses continue for analog systems and subsystems.
In such digital systems, and in analog systems, there is often a need for a delay in time between the occurrence of one event indicated by a signal and the start of some other event, or at least during some acknowledgment of the occurrence of the first event. In these situations, and other delay situations, a delay arrangement of some kind must be provided in such a system.
Various kinds of delay systems have been used in the past. For instance, an analog delay system has been used in which a logic level or signal is delayed in time by the amount of time such a signal takes to propagate along a discrete delay line. Currently, however, many delay systems must be amenable to inclusion in a monolithic integrated circuit chip for convenience, cost and performance reasons. Today's systems meeting this requirement have delays provided therein often based on either the time taken by a current source to linearly charge a capacitor in an analog approach, or by the time taken to count a number of pulses provided by a timing, or clocking, circuit in a digital implementation.
The former method, depending on capacitors and current sources, has the disadvantage of often requiring a relatively large amount of space in an integrated circuit chip, and the further disadvantage of needing accurate, or accurately compensable, comparators, current sources, and capacitors obtainable only through fabrication processes which exhibit variations and which must operate in a system that maintains accuracy over a substantial temperature range. The latter scheme of counting clock pulses requires the availability of a sufficiently accurate source of such clocking pulses. Thus, a desirable delay system without resort to large capacitors, accurate components or an accurate source of clock pulses is desired.